This invention relates generally to techniques of producing composite or laminated structures of synthetic resins. More particularly, the invention relates to a method and die construction for forming a composite or laminated tubular structure by efficiently bonding together a plurality of molten thermoplastic synthetic resin materials within a die.
In recent years, there has been continuing progress in the laminating of a wide variety of plastic materials in order to effectively utilize the characteristics of the resins and to produce better packaging materials, and processes for forming tubular laminated structures by compositing melt extruded resins within dies are being widely practiced. The majority of the laminating dies for this purpose are of a construction wherein, a first resin is caused to flow past a cross head part or the arm part of a spider die to be formed into a tubular structure, and thereafter a second resin from the outside or from within a core is caused to merge in a tubular state with the tubular structure of the first resin thereby to form a laminated tubular structure.
The production of tubular structures in this manner is accompanied by the problem of obtaining a uniform flow of thin layers of resins and the problem of adhesiveness between adjacent layers. For solving the former problem, various proposals have been made to provide the die with the capability of adjusting deviations in layer thickness for each layer in order to render the thickness of each layer uniform, but with respect to the layer thickness, there is a natural limit to the mechanical working precision. For improving the adhesiveness between adjacent resin layers in order to solve the latter problem, this can be accomplished by appropriately selecting the resin materials, but there is a natural limit to the range of materials. Alternatively, various proposals for admixing other resins for the purpose of improving the adhesiveness have been made, but such measures give rise to other problems such as impairment of transparency and increasing of the oxygen permeability, whereby the original desirable properties of the resins are lost.
In contrast to these measures, the insertion of a separate adhesive layer between adjacent layers is effective. In this case, however, by the ordinarily used method described hereinbefore, the die construction becomes complicated, and, although the insertion of a thin adhesive layer in a uniform manner is required, this is difficult, the layer becoming unnecessarily thick. As a consequence, there arise further problems such as a lowering of the adhesiveness between adjacent layers and impairment of transparency.